To prevent contamination of potable water supplies from downstream contaminants, back flow prevention valves are commonly employed in the water line and are generally required by applicable law. Such valves are typically installed in water lines downstream of the water meter and act to prevent fluid flow back through the valve which would otherwise occur if the downstream pressure exceeded the upstream pressure. While the back flow prevention valves currently in use generally accomplish their intended purpose, they have been found to have several inherent shortcomings. Such valves typically employ diaphragms which are subject to wear and bursting at high pressure. The currently available devices also generally employ circuitous flow passageways which cause excessive pressure drops rendering these valves poorly suited for high volume flow. The solution to this problem has been to oversize the valves which necessarily increases the cost of construction. The problem of excessive pressure drop is aggravated by the insensitivity of these valves, resulting in a slow opening response which typically occurs only when a 30 to 40 psi pressure differential is created across the valve. The light spring loads and flat seals employed in these devices also inhibit high volume flow and require a large head force to open the valve fully which, in combination with the rapid closing found in such valves, creates undesirable water hammering.
In addition to the above, the back flow prevention valves currently in use uniformly have inadequate reduced pressure zones across the valves which are necessary to maintain a downstream pressure within the valve which is below the upstream pressure to prevent upstream siphoning and resulting contamination. This is a particularly acute problem in high hazard applications such as metal plating acid tanks and the like which present substantial hazards to potable water supplies. In addition to providing inadequate safeguards against upstream siphoning, the inadequate pressure drops in the reduced pressure zones of the prior art, which are typically about only 1 to 5 psi, create periodic unloading and loading of the valves in response to common fluctuations in line pressure. This further inhibits high volume flow and results in frequent undesirable water dumping or leakage. These problems are illustrative of the shortcomings found in the back flow prevention valve heretofore available.
In view of the above, it would be highly desirable to provide a compact, responsive and durable back flow prevention valve highly suitable for high hazard applications and one which is capable of handling both high and low volume flow with minimal restriction and pressure loss. It would also be highly desirable to provide a back flow prevention valve which maintains a substantially greater pressure drop in the reduced pressure zone than found in current valves to provide improved protection against upstream siphoning. Finally, it would be desirable to substantially eliminate the effect of line pressure fluctuations and reduce water hammering. Such a valve is disclosed and claimed herein.